1. Field of the Invention
The present invention relates to an image forming apparatus and method, and more particularly to an image forming apparatus and method that transfers visible images carried by each of a plurality of latent image carriers to a recording sheet after superimposing those visible images on a surface of an endless moving belt, or superimposes visible images carried by each of a plurality of latent image carriers on a recording sheet held on the surface of the endless moving belt.
2. Description of the Related Art
Known image forming apparatuses move an endless intermediate transfer belt as a surface endless moving member tensioned by a plurality of tension rollers. Four first transfer nips are formed on the front surface of the intermediate transfer belt by contacting four photoconductors, yellow (Y), magenta (M), cyan (C), and black (K) that form toner images of each color. After superimposing Y, M, C, and K toner images formed on the surface of each photoconductor on the intermediate transfer belt at each of Y, M, C, and K first transfer nip, the image forming apparatus transfers the superimposed toner image to the recording sheet en bloc. Accordingly, a full color image is formed on the recording sheet.
Alternatively, the image forming apparatus may use a transfer belt that transfers a recording sheet held on its surface that moves endlessly instead of using the intermediate transfer belt. This type of image forming apparatus transfers Y, M, C, and K toner images formed on each of Y, M, C, and K photoconductor to the recording sheet on the transfer belt directly, and produces a full color image.
Like these image forming apparatuses described above, a type that transfers toner images formed on each of a plurality of photoconductors to a surface of an endless moving member like a belt or a recording sheet held on the surface of the endless moving member is called a tandem type. Tandem-type image forming apparatuses have an advantage in that productivity (number of sheets of recording paper that can be printed per unit time) is improved substantially. By contrast, tandem type image forming apparatuses have a disadvantage in that color misalignment (registration misalignment), a phenomenon in which toner images of each color are transferred in misaligned position with each other due to positional misalignment and dimensional tolerances of photoconductors and optical writing units, etc., in image forming units for each color, is caused easily. Consequently, it is necessary to execute color misalignment correcting control (in other words, registration control) to correct the color misalignment.
The following process for color misalignment correcting control is well-known. First, a test pattern image that includes test toner patterns for each color for detecting color misalignment is formed on an intermediate transferring belt. Subsequently, amount of color misalignment (amount of registration misalignment) is calculated based on results of detecting positions of test toner images for each color in the test pattern image by sensors. Lastly, optical paths in optical systems for each color, image writing positions for each color, and pixel clock frequency are corrected based on calculated amount of color misalignment (amount of registration misalignment).
However, the color misalignment correcting control described above has two main issues. First issue is that it is necessary to adjust positions of optical paths in optical system for each color with each other by moving a mirror in an optical path and part of optical system including a light source and a f-θ lens mechanically in order to correct optical paths in the optical system, and that drives up cost since it is necessary to provide precision moving parts to do that. Furthermore, it is impossible to correct color misalignment in a short time interval since it takes a relatively long time to finish adjusting positions of optical paths after starting color misalignment correcting control.
Second issue is that it is difficult to maintain high-quality imaging just after finishing color misalignment control for a long period of time since sometimes the amount of color misalignment (amount of registration misalignment) changes with time because of a deformation in the optical system and supporting parts due to a change of internal temperature.
A well-known image forming apparatus can resolve the first issue described above (e.g., JP-H8-85236-A). This image forming apparatus transfers toner images of photoconductors for each color on a recording sheet held on a transferring belt that moves endlessly. The image forming apparatus executes following the processes at predefined timing, including transferring a test pattern image that includes test toner images for each color on the transferring belt and acquiring information on forming coordinates of test toner images for each color based on results of detecting test toner images for each color in the test pattern image by sensor. Subsequently, the image forming apparatus converts automatically output coordinate positions of image data for each color into output coordinate positions with corrected registration misalignment based on the amount of registration misalignment decided by the information on forming coordinates and pre-stored standard position coordinates.
Another image forming apparatus that can resolve the first issue is also well-known (e.g., JP-2005-274919-A). This image forming apparatus corrects positions in a main scanning direction and a sub-scanning direction of image data for each color in output coordinates based on results of detecting positions of test toner images for each color in a registration alignment detecting pattern formed on an intermediate transferring belt. Furthermore, one or more of scale in main scanning direction in output coordinate, partial scale in main scanning direction, scale in sub-scanning direction, partial scale in sub-scanning direction, lead skew, side skew, lead linearity, and side linearity are variable.
Also, a well-known image forming apparatus that can resolve the second issue described above executes color misalignment correcting control in case internal temperature changes at a certain amount and executes color misalignment correcting control repeatedly after elapse of a certain amount of time.
However, although this image forming apparatus can form high-quality image just after executing color misalignment correcting control, the image forming apparatus does not take into account amount of color misalignment that changes over time. Also, this image forming apparatus cannot reduce color misalignment due to positional error in direction of movement of photoconductor surface generating in a single rotation cycle of photoconductor at the optical writing position (periodic positional error). More particularly, there is a slight eccentricity in the rotation axis of the photoconductor and a photoconductor gear that rotates with the rotation axis. Due to this eccentricity, linear velocity fluctuates in a sine carve of one period for one rotation of the photoconductor at the optical writing position where optical writing is executed on the photoconductor. Due to this linear velocity fluctuation, periodic positional error is generated in a sine curve of one period for one rotation of photoconductor (periodic positional deviation fluctuation curve) at the optical writing position. If amplitude of positional fluctuation curve (=amount of eccentricity) that characterizes periodic positional error is different from each other or phase differences of positional fluctuation curve do not match for Y, M, C, and K photoconductors, relative position deviation is generated on toner images for each color due to periodic positional error and color misalignment is generated. Therefore, the image forming apparatus cannot form high-quality images.